Microtonal music, music using tones in intervals that differ from the standard semitones (half steps) of a tuning system or scale. In the division of the octave established by the tuning system used on the piano, equal temperament, the smallest interval (e.g., between B and C, F and F♯, A♭ and A) is the semitone, an interval also measured as 100 cents. There are thus 12 equal semitones, or 1,200 cents, to the octave; these in sequence constitute the chromatic scale. Western tuning systems that were more common before about 1700 divided the octave into semitones of varying size.

Although the term microtonal suggests that such music departs from a norm, most of the world’s music, of both past and present times, uses intervals greater or smaller than 100 cents. South Asian music theory posits a scale of 22 unequal intervals to the octave; although, in practice, a chromatic scale of 100-cent intervals is used, ornaments use intervals of smaller size. In Indonesian music, intervals of many sizes appear, including those of the slendro scale, which sometimes divides an octave into five equal intervals of roughly 240 cents each. Essential in Middle Eastern music are intervals of 150 cents (three-quarter tones) and 250 cents (five-quarter tones), along with half and whole tones (100 and 200 cents); some 20th-century Middle Eastern theory builds intervals from combinations known in ancient Greek theory as comma (24 cents) and limma (90 cents).

Some Western composers and music theorists have suggested the use of microtonal intervals derived from the octave of 100-cent half tones—e.g., intervals of a quarter tone (50 cents), 6th tone (33.3 cents), 12th tone (16.7 cents), and 16th tone (12.5 cents). In this last case, the octave would consist of 96 equal divisions, and the modern semitone would equal eight of them in sequence; e.g., between B and C would lie eight equal 16th-tone intervals.

Influenced by European tuning systems used before 1700 and by non-Western musics, many composers in Europe and North America began to experiment with microtonal structures soon after 1900. Most prominent was the Czech composer Alois Hába, who wrote many pieces, including operas, using quarter-tone and sixth-tone scales; he designed instruments to play the music, and he established at the Prague Conservatory a department of microtonal music (which existed, except for a period during World War II, from 1934 until 1949). Among the well-known Western composers to incorporate microtonal material into their music were Charles Ives, Harry Partch, Henry Cowell, John Cage, Benjamin Johnston, Henk Badings, Karlheinz Stockhausen, and Krzysztof Penderecki.

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...a simple combination of notes, octaves, fifths, and fourths, usually below the melody notes, may be used as an ornamentation. Among the elements contributing to the enrichment of the melody are microtonality (the use of intervals smaller than a Western half step or lying between a half step and a Western whole step) and the variety of intervals used. Thus, the three-quarter tone, introduced...